Affinity chromatography of partially adenylylated glutamine synthetase on Cibacron Glue - Sepharose columns was used to separate molecular species of hybrid enzyme forms that differ in the number of adenylylated subunits. The Mg 2 ion supported biosynthetic specific activity of these hybrids, is directly proportional to the number of unadenylylated subunits per molecule; whereas the Mn 2 ion supported activity is directly proportional to the number of adenylylated subunits. The affinity of the adenylylated subunits for glutamate decreases as the number of adenylylated subunits per molecule increases. Anti-AMP antibodies prepared against adenylylated serum albumin and purified by affinity chromatography will react with adenylylated but not with unadenylylated subunits. The rate of reaction and extent of enzyme precipitations are functions of the state of adenylylation of the enzymes as well as the variations in the conformational state elicited by divalent cation interactions.